The present invention relates generally to a color cathode ray apparatus, and more particularly to a color cathode ray tube apparatus wherein an elliptic distortion of a beam spot at a peripheral portion of a screen is reduced and thereby an image with high quality is displayed.
In general, a color cathode ray tube (CRT) apparatus has a vacuum envelope comprising a panel and a funnel. Three electron beams are emitted from an electron gun assembly disposed in a neck of the funnel. The three electron beams are deflected by horizontal and vertical deflection magnetic fields generated by is a deflection yoke. The deflected beams are then guided through a shadow mask onto a phosphor screen provided on an inner surface of the panel. The phosphor screen is scanned horizontally and vertically by the three electron beams, and thus a color image is displayed on the phosphor screen.
A self-convergence in-line type color cathode ray tube in which an in-line type electron gun assembly is built, in particular, has widely been used as the above color CRT apparatus. In the in-line type electron gun assembly, electron guns are horizontally arranged to emit three in-line electron beams consisting of a center beam and a pair of side beams in the same horizontal plane. In the self-convergence type color CRT, its deflection yoke generates non-uniform magnetic fields, i.e. a pin-cushion-shaped horizontal deflection magnetic field and a barrel-shaped vertical deflection magnetic field, and the in-line three electron beams self-converge on the screen.
Electron gun assemblies for emitting three in-line electron beams may have various structures. There is known an electron gun assembly of a bipotential (BPF) type DACF (Dynamic Astigmatism Correct and Focus) system. The electron gun assembly of the BPF type DAF system, as shown in FIG. 1, comprises three in-line cathodes K and first to fourth grids G1 to G4 arranged in the named order from the cathode K side toward a phosphor screen. The third grid G3 is comprised of two divisional segment electrodes G31 and G32. The grids G1, G2, G31, G32 and G4 are integrally constructed such that each has three in-line electron beam passage holes for passing electron beams and the positions of these holes correspond in position to three cathodes K.
In this electron gun assembly, a voltage of about 150V is applied to each cathode K. The first grid G1 is grounded, and a voltage of about 600 to 800V is applied to the second grid G2. A voltage of about 6 kV is applied to the first segment electrode G31 of the third grid G3. The second segment electrode G32 is supplied with a dynamic voltage increasing in synchronism with deflection of an electron beam by the deflection yoke, which dynamic voltage being added to a reference voltage applied to the first segment electrode G31. A high voltage of about 26 kV is applied to the fourth grid G4.
In the electron gun assembly, with the application of such voltages, the cathodes K and first and second grids G1 and G2 generate electron beams and constitute a three-pole (triple-pole) unit for forming an object point on a main lens (described below). The second grid G2 and the first segment electrode G31 of the third grid G3 constitute a prefocus lens for preliminarily focusing the electron beams from the triple-pole unit. The first and second segment electrodes G31 and G32 constitute a quadruple-pole lens for horizontally focusing and vertically diverging electron beams when they are deflected. The second segment electrode G32 and fourth grid G4 constitute a high-potential (BPF) type main lens for finally focusing the electron beams on the phosphor screen.
In this electron gun assembly, when the electron beams are directed to the center of the screen without deflection, the quadruple-pole lens is not formed between the first and second segment electrodes G31 and G32. The electron beams from the triple-pole unit are preliminarily focused by the prefocus lens and focused on the center of the screen of the main lens.
On the other hand, when the electron beams are deflected toward the periphery of the screen, the voltage of the second segment electrode G32 is increased in accordance with the amount of deflection of the electron beams and the quadruple-pole lens for horizontally focusing and vertically diverging electron beams is formed between the first and second segment electrodes G31 and G32. At the same time, with the increase in voltage of the second segment electrode G32, the power of the main lens formed at the second segment electrode G32 and fourth grid G4 is decreased. Thereby, when the electron beams are deflected toward the periphery of the screen, the electro-optical distance between the electron gun assembly and the phosphor screen increases and an image point will form at a long distance. Accordingly, the magnification of the lens varies to cancel a deflection aberration occurring due to the fact that the horizontal deflection field generated by the deflection yoke has a pin-cushion shape and the vertical deflection field has a barrel-shape.
In the meantime, in order to enhance the image quality of the color CRT, it is necessary to enhance the focusing characteristics of the entire screen. However, in an in-line type color CRT having a regular electron gun assembly for emitting three in-line electron beams, as shown in FIG. 2A, a beam spot at a peripheral portion of the screen is distorted to a horizontal elliptic shape 1b (horizontal deformation) due to a deflection aberration and a vertical blur 2 occurs, although a beam spot 1b at a central portion of the screen has a substantially circular shape.
On the other hand, in the in-line type color CRT having the electron gun assembly, as shown in FIG. 1, the blur 2 can be eliminated and the focusing characteristics can be enhanced, as shown in FIG. 2B. This electron gun assembly adopts the DACF system, and the low-voltage side electrode constituting the BPF type main lens is divided into a plurality of segment electrodes and these segment electrodes form the four-pole lens in accordance with the amount of deflection of electron beams, thereby to compensate the deflection aberration. Even in the electron gun assembly with this structure, however, the horizontal deformation of the beam spot 1b at the peripheral portion of the screen cannot be eliminated. As a result, a moire occurs due to an interference between the electron beams and the beam passage holes in the shadow mask, and displayed characters, etc. on the screen becomes difficult to view.
In a method of solving the above problem, in the above-described electron gun assembly, as shown in FIG. 3, non-circular electron beam passage holes 4, each having a horizontal long axis, are formed in that surface of the second grid G2, which face the first segment electrode G31 of third grid G3. In the electron gun assembly with this structure, the horizontal focusing power of the prefocus lens constituted by the second grid G2 and the first segment electrode G31 is weaker than the vertical focusing power thereof, and a horizontal imaginary object point size is reduced and a vertical imaginary object point size is increased. As a result, as shown in FIG. 2C, the beam spot la at the central portion of the screen is vertically elongated and the horizontal deformation of the beam spot 1b at the peripheral portion of the screen is reduced. Thus, the moirxc3xa9 due to an interference between the electron beams and the beam passage holes in the shadow mask can be prevented.
In this electron gun assembly, as the depth of the non-circular recess 4 with the horizontal long axis, which is formed in the second grid, increases, the horizontal deformation of the beam spot 1b at the peripheral portion of the screen can be reduced more effectively. As a result, however, the vertical length of the beam spot 11 at the central portion of the screen is increased and the vertical dimension of the beam spot increases. Consequently, the resolution at the central portion of the screen deteriorates.
As means for solving this problem, FIG. 3 shows an electron gun assembly wherein an auxiliary grid Gs having vertically or horizontally elongated non-circular electron beam passage holes is disposed between the second grid G2 and the first segment electrode G31 of the third grid G3. The auxiliary grid Gs is supplied with a dynamic voltage increasing or decreasing in synchronism with the deflection of electron beams.
With this structure, the horizontal focusing and vertical focusing of the prefocus lens formed by the second grid G2 and first segment electrode G31 can be dynamically altered. Thereby, when the electron beams are not deflected and are directed to the central area of the screen, the horizontal focusing of the prefocus lens is equalized to the vertical focusing. In addition, when the electron beams are deflected toward the periphery of the screen, the prefocus lens is provided with such an astigmatism that the horizontal focusing is weak and the vertical focusing is strong, and the horizontal imaginary object point size is reduced while the vertical imaginary object point size is increased. Thus, a color CRT displaying high-quality images can be provided wherein the vertical size of the beam spot at the peripheral portion of the screen is increased without degradation in resolution at the central portion of the screen, and the horizontal deformation at the peripheral portion of the screen is reduced and the focusing is made uniform over the entire area of the screen.
In actuality, however, in order to obtain a desired electron beam divergence angle and a desired imaginary object point size with the above electron gun assembly, a relatively high dynamic voltage of 1.5 to 3 kv needs to be applied to the auxiliary grid Gs. The reason is that the auxiliary grid Gs faces the first segment electrode G31 of third grid G3 to which a relatively high voltage of about 6 kV is applied and if the voltage to the auxiliary grid Gs is decreased, a shift of potential from the first segment electrode G31 to the auxiliary grid Gs becomes too great and the astigmatism of the prefocus lens becomes too strong.
As has been described above, in order to apply a relatively high dynamic voltage to the auxiliary grid Gs, a driver circuit for generating a relatively high dynamic voltage is required and the cost for circuit elements increases.
In order to enhance the image quality of the color CRT, it is necessary that the good focusing state be maintained over the entire screen and an elliptic distortion of the beam spot be decreased.
In this respect, with the conventional BPF-type DACF-system electron gun assembly, a dynamic voltage increasing in synchronism with deflection of electron beams is applied to the low-voltage-side electrode forming the BPF-type main lens, thereby forming a four-pole lens and varying the power of the main lens. Thus, a vertical blur of the beam spot at the peripheral portion of the screen due to the deflection aberration can be eliminated and the focusing characteristics enhanced. However, with this electron gun assembly, the horizontal deformation of the beam spot at the peripheral portion of the screen cannot be prevented, and a moirxc3xa9 occurs due to an interference between the electron beams and the beam passage holes in the shadow mask. Consequently, displayed characters, etc. on the screen become difficult to view.
In order to solve the problem of horizontal deformation of the beam spot at the peripheral portion of the screen, there has been proposed an electron gun assembly wherein non-circular recesses, each having a horizontal long axis, are formed in that surface of the second grid, which face the first segment electrode of the third grid. According to this electron gun assembly, the horizontal deformation of the beam spot at the peripheral portion of the screen is reduced and the moirxc3xa9 due to an interference between the electron beams and the beam passage holes in the shadow mask can be prevented. However, the beam spot at the central portion of the screen is vertically elongated. Moreover, as the depth of each non-circular recess with the horizontal long axis, which is formed in the second grid, increases, the horizontal deformation of the beam spot at the peripheral portion of the screen can be reduced more effectively, and the vertical length of the beam spot at the central portion of the screen is increased. Consequently, the resolution at the central portion of the screen deteriorates.
In other words, with this electron gun assembly, if importance is placed on the clearness of image at the central portion of the screen, the image quality at the peripheral portion of the screen will deteriorate. If importance is placed on the clearness of image at the peripheral portion of the screen, the image quality at the central portion of the screen will deteriorate. Consequently, in the color CRT having the electron gun assembly with the above structure, the focusing over the entire screen cannot be performed satisfactorily, and less desirable designing needs to be done.
In order to solve the above problem, there has been proposed an electron gun assembly wherein an auxiliary grid having vertically or horizontally elongated non-circular electron beam passage holes is disposed between the second grid and the first segment electrode of the third grid. This auxiliary grid is supplied with a dynamic voltage increasing or decreasing in synchronism with the deflection of electron beams.
With this structure, a color CRT displaying high-quality images can be provided wherein the vertical size of the beam spot at the peripheral portion of the screen is increased without degradation in resolution at the central portion of the screen, and the horizontal deformation at the peripheral portion of the screen is reduced and the focusing is made uniform over the entire area of the screen. With this electron gun assembly, however, a relatively high dynamic voltage of 1.5 to 3 kV needs to be applied to the auxiliary grid, and the cost for the driver circuit increases.
The object of the present invention is to provide a color CRT capable of performing uniform focusing over the entire screen with a relatively low dynamic voltage.
(1) A color cathode ray tube apparatus has an electron gun assembly for generating three in-line electron beams traveling in a single plane. The electron gun assembly has a plurality of electrodes including cathodes for generating the three electron beams and constituting a triple-pole unit, first and second grids disposed successively from the cathode side toward a phosphor screen side, and a third grid disposed adjacent to the second grid, the third grid forming a lens for focusing the electron beams from the triple-pole unit onto the phosphor screen. The three electron beams emitted from the electron gun assembly are deflected by non-uniform horizontal and vertical deflection magnetic fields generated by a deflection yoke and are self-converged. First and second auxiliary grids are disposed between the second grid and the third grid. A dynamic voltage, which varies in synchronism with deflection of the electron beams, is applied to the first auxiliary grid. A fixed voltage is applied to the second auxiliary grid. The second grid, first and second auxiliary grids and third grid form an electron lens such that a higher astigmatism is provided by focusing in a direction perpendicular to a direction of arrangement of the three electron beams than by focusing in the direction of arrangement of the three electron beams and the degree of the astigmatism is dynamically varied in accordance with the dynamic voltage applied to the first auxiliary grid.
(2) In the color cathode ray tube apparatus according to aspect (1), a dynamic voltage obtained by superimposing a voltage increasing in synchronism with the deflection of the electron beams to a voltage substantially equal to a voltage applied to the second grid is applied to the first auxiliary grid.
(3) In the color cathode ray tube apparatus according to aspect (1), a voltage equal to a voltage applied to the second grid is applied to the second auxiliary grid.
(4) In the color cathode ray tube apparatus according to aspect (1), the first auxiliary grid has electron beam passage holes for passage of the three electron beams, each of the electron beam passage holes being formed non-circular such that a dimension thereof in a direction perpendicular to a direction of arrangement of the three electron beams is greater than a dimension thereof in the direction of arrangement of the three electron beams.
(5) In the color cathode ray tube apparatus according to aspect (1), the second auxiliary grid has circular electron beam passage holes for passage of the three electron beams.
(6) In the color cathode ray tube apparatus according to aspect (1), the second auxiliary grid has electron beam passage holes for passage of the three electron beams, each of the electron beam passage holes being formed non-circular such that a dimension thereof in a direction perpendicular to a direction of arrangement of the three electron beams is different from a dimension thereof in the direction of arrangement of the three electron beams.
(7) In the color cathode ray tube apparatus according to aspect (1), that surface of the second grid, which faces the first auxiliary grid, has non-circular recesses each having a major axis in a direction of arrangement of the three electron beams or a groove elongated in the direction of arrangement of the three electron beams, independently of three beam passage holes formed in the second grid.
(8) In the color cathode ray tube apparatus according to aspect (1), the second grid has circular holes for passage of the three electron beams, the first auxiliary grid has holes for passage of the electron beams, each of which holes is formed non-circular such that a dimension thereof in a direction perpendicular to a direction of arrangement of the three electron beams is greater than a dimension thereof in the direction of arrangement of the three electron beams, and the second auxiliary grid has circular holes for passage of the electron beams. When a diameter of each of the holes in the second auxiliary grid is xcfx86G2, a dimension of each of the holes in the first auxiliary grid in the direction perpendicular to the direction of arrangement of the three electron beams is xcfx86Gs1V, a dimension of each of the holes in the first auxiliary grid in the direction of arrangement of the three electron beams is xcfx86Gs1H, and a diameter of each of the holes in the second auxiliary grid is xcfx86Gs2, the following relationship is established:
xcfx86G2xe2x89xa6xcfx86Gs1H less than xcfx86Gs2xe2x89xa6xcfx86Gs1V.
(9) In the color cathode ray tube apparatus according to aspect (1), the third grid is divided into first and second electrodes, and a dynamic voltage varying in synchronism with deflection of the electron beams is applied to the second electrode disposed apart from the second auxiliary grid.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.